The present invention relates to improvements in titanium alloys. More particularly, it relates to multicomponent titanium alloys which are improved by the addition of boron thereto in a prescribed compositional and processing relationship.
It is known that there is a continuing need for titanium alloys with improved performance at elevated temperatures. Further, it is known that titanium alloys with high aluminum content have good elevated temperature properties but suffer in that they have poor room temperature ductility. Any modification of titanium alloys with high aluminum to increase the low temperature ductility would be very beneficial in that it would permit new uses of such high aluminum titanium alloys in demanding applications such as in jet engines.
It is known in the art that conventional high temperature titanium alloys have been limited in their high temperature capabilities because of the difficulty or impossibility of adding alloying elements beyond a given level without room temperature embrittlement. This was described originally in a January 1957 article in the Journal of Metals, entitled "Embrittlement of Ti-Al Alloys in the 6-10% Al Range", authored by Crossley and Carew, at pages 43-46, and describing levels above about 6 weight percent causing brittle behavior which behavior was reported to be made worse by thermal exposure.
Development work has proceeded along the path recognized by Crossley and Carew until it was recognized that any strengthening; element which behaves like aluminum will cause embrittlement. Consequently, it was recognized in the art that the elements--tin, zirconium and oxygen--all had to be controlled to lower concentrations in order to avoid the embrittlement. The most modern current titanium base alloys, such as Ti-1100, described by Bania in "An Advanced Alloy for Elevated Temperatures" in the March 1988 issue of the Journal of Metals, on pages 20-22, and in U.S. Pat. No. 4,738,822, entitled "TITANIUM ALLOY FOR ELEVATED TEMPERATURE APPLICATIONS; WITH ALUMINUM, ZIRCONIUM, MOLYBDENUM, SILICON, AND IRON", contain only about 6 weight percent aluminum, evidently for the same reason.
An alternative approach to alloying was described by Blackburn and Smith in U.S. Pat. No. 4,292,077, entitled "TITANIUM ALLOYS OF THE Ti.sub.3 Al TYPE" and also in U.S. Pat. No. 4,716,020 entitled "TITANIUM ALUMINUM ALLOYS CONTAINING NIOBIUM, VANADIUM AND MOLYBDENUM". It was pointed out in these patents that titanium base alloys with higher aluminum content, above the amount that cause formation of alpha two (Ti.sub.3 Al) as the major phase, and with additions of refractory elements such as niobium (columbium) or vanadium, can result in production of a finite volume fraction of a ductilizing beta phase. However, the limitation of this approach is that the beta phase, required for the room temperature ductility, seriously degrades the strength of such alloys at temperatures of 650.degree. C. and above.
It is known that boron has low solubility in titanium and the effects of boron additions to titanium base alloys are described in a reference text entitled "Binary Alloy Phase Diagrams" published by the American Society of Metals (1986) and edited by editor-in-chief Thaddeus B. Massalski and editors Joanne L. Murray, Lawrence H. Bennett, and Hugh Baker. According to this reference, boron additions to titanium base alloys would be expected to result in precipitates of boride phase and would, accordingly, not be expected to have a modifying effect on a titanium matrix microstructure.
The use of boron to form a second phase compound is well-known in various classes of titanium alloys that have been prepared by conventional solidification and thermomechanical processing techniques. Jaffee, Maykuth, and Ogden in U.S. Pat. Nos. 2,596,489 and 2,797,996 describe alpha and alpha plus beta titanium alloys which would contain boron at a sufficiently high level that it would form a boride dispersed phase. Jaffee in U.S. Pat. No. 2,938,789 describes beta titanium matrix compositions with boride or silicide phases. Brooks, Brown, and Jepson in U.S. Pat. No. 3,199,980, describe titanium alloys with boride or carbide precipitates. Evans and Smith in U.S. Pat. No. 3,340,051 describe a titanium-chromium alloy with boron at a sufficiently high level that it contains a dispersed boride phase, and in U.S. Pat. No. 3,399,059 they describe titanium-molybdenum-vanadium beta matrix compositions containing boron. In contrast, the titanium alloy compositions and processing of my invention yield modified alpha matrix phase microstructures and improved low temperature ductility via using boron at lower levels of concentration and rapidly solidifying the alloy compositions to prevent the formation of dispersed borides.
Boron containing alloys at lower levels were described by Itoh, Miyauchi, Sagoi, and Watanabe in U.S. Pat. No. 4,253,873. They describe an optional addition of boron at a sufficiently low level that it might not form borides. However, the alloys of their invention have such low levels of the alpha promoter aluminum that embrittlement by Ti.sub.3 Al is not an issue, and such high levels of the beta promoting elements chromium and molybdenum that the alloy has either a retained beta matrix or a chromium eutectoid microstructure. In contrast, the alloys and processing of my invention relate to improving the ductility of high aluminum alloys and to modifying an alpha plate microstructure.
Rapid solidification of boron-containing titanium alloys was described by Vordahl in U.S. Pat. Nos. 3,622,406 and 3,379,522. These alloy compositions were chosen to have a sufficiently high level of boron that it would form dispersoids. The purpose of rapid solidification was to refine these dispersoids.
In a September 1983 article in the Journal of Metals, pages 21-27, entitled "Rapid Solidification Processing of Titanium Alloys", S. M. L. Sastry, T. C. Peng, T. J. Meschter, and J. E. O'Neal reported that rapid solidification of boron containing compositions was expected to result in a refined array of borides potentially useful as dispersoids. They taught this further in U.S. Pat. No. 4,639,281.
Similarly, in an article entitled "Control of Beta-Grain Growth Via The Powder Metallurgy Route In A Ti-6Al-4V Alloy" by H. Octor, S. Naka, M. Marty, and A. Walder, appearing in a reference entitled "Annealing Processes, Recovery, Recrystallization, and Grain Growth", published by Riso of Denmark (Dec. 8-12, 1986), it was pointed out that boron present in a titanium base alloy as a precipitate might be expected to refine the beta grain size. In this work, additions of base boron were made to the Ti-6Al-4V alloy by blending powders. Boron was observed to prevent beta grain growth. In this article and in the previous article, no modification of the alpha titanium microstructure was observed or reported.
It is known that the presence of boron and conventionally solidified titanium alloys and in titanium alloy weldments has a negative impact on low temperature ductility. Two articles on the behavior of an alpha titanium alloy containing boron reveal that there is no modification on the alpha plate microstructure and there is degradation in the mechanical properties at room temperature where the boron is present. The first article is entitled "Boron Induced Toughness Loss in Ti-6Al-2Nb-1Ta-0.8Mo", by H. Inouye and S. A. David, and an article entitled "The Effect of Boron on Weldment Microstructures In The Ti-6Al-2Nb-1Ta-1Mo Alloy", by R. E. Lewis, W. C. Kuhns, F. A. Crossley, I. L. Kaplan, and W. E. Lukens. Both articles appeared in the Proceedings of the Fifth International Conference on Titanium, in Munich, F.R.G. (Sept. 10-14, 1984) as edited by G. Lutjering, U. Zwiker, and W. Bunk.
By contrast to the findings reported in the literature, I have found that a titanium base alloy can be provided which has improved low temperature strength and ductility and which also possesses good high temperature strength and that this can be accomplished by additions of boron combined with rapidly solidification of high aluminum content alloys to modify the alpha plate microstructure.